Crushable bead on lead finger side surface to improve moldability

ABSTRACT

A surface mount semiconductor package employs locking elements for locking a plastic housing to a metal pad on which a semiconductor device is mounted. The package includes terminals having elongated crushable beads on their side surfaces adjacent the portions of the terminals just outside the plastic housing. The beads are crushed inwardly by a molding tool when it closes to provide a seal which prevents the molding plastic from bleeding out and over the sides of the terminals which extend beyond the housing and which could interfere with solder connection to the terminals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority to: (i) U.S.Provisional Patent Application Ser. No. 60/025,458, filed Sep. 5, 1996,entitled CRUSHABLE BEAD ON LEAD FINGER SIDE SURFACE TO IMPROVEMOLDABILITY; and (ii) Provisional Patent Application Ser. No.60/025,832, filed Sep. 5, 1996, entitled IMPROVED SURFACE-MOUNT HIGHPOWER SEMICONDUCTOR PACKAGE AND METHOD OF MANUFACTURE.

FIELD OF THE INVENTION

The present invention relates to semiconductor device packages and leadframes therefor, and more specifically relates to a high powersemiconductor device adapted for surface mounting.

BACKGROUND OF THE INVENTION

Packages for high power semiconductor devices which can be surfacemounted on an insulated metal substrate (IMS) or other flat supportboard surface are well known. One such package is shown in U.S. patentapplication Ser. No. 08/583,219, filed on Jan. 4, 1996, entitledSURFACE-MOUNT SEMICONDUCTOR PACKAGE which is incorporated herein byreference. These devices are very well adapted for surface mounting tothe conductive patterns of flat support boards such as an IMS structure(a thick copper or aluminum substrate covered by a thin insulation filmwhich has a thin patternable copper or other conductive solderable uppersurface).

The present invention is an improvement of that device, making it moreefficient and more easily manufacturable.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a novel lead framewhich can receive one or more semiconductor die such as power IGBT die,IGBT die, Schottky diode die and mixtures thereof on a central, flat padsection. The die are interconnected by the pad at their bottom surfaces,and at their tops by appropriate wire bonds. The lead frame has twopower terminals, which may be interconnected at two adjacent corners ofa rectangular package. The power terminals are accessible for connectionexternally of a flat plastic molded housing which encloses the top andsides of the central lead frame pad. A plurality of control pins orterminals, which initially are a part of the lead frame but are isolatedfrom the heat sink pad after molding of the housing, extend from theside of the housing which is opposite the side containing the powerterminal.

There are at least two closely spaced control terminals or pins whichcan be wire bonded to the gate and cathode or current sense terminals ofthe die within the housing. A remote third terminal (from the closelyspaced first and second control terminals) is also available forconnection to some other terminal, for example, the gate terminal of athyristor die if such a die is contained within the housing.

The lead frame is preferably a single gauge conductive sheet. Theterminals extending through the borders of the molded housing can bepartially vertically offset to provide an improved plastic lock to thelead frame. The bottom surfaces of the terminals and the lead frame padare in a common plane. The main heat sink pad can have parallel slotstherethrough on opposite sides of the die on the pad to provide afurther plastic lock to the molded housing. Shallow dovetail grooves canextend from an interior edge of these slots to also provide improvedplastic locking.

The surface of the pad may have a waffled or dimpled surface to improvesoldering of the bottom die surface electrodes to the pad. In accordancewith a feature of the invention, the bottom surface of the pad which isto be surface mounted to a heat sink or conductive pattern of an IMSboard can also be waffled to improve the solder down of the pad to theheat sink and to avoid solder voids due to a concavity in the lead framebottom.

The bottom of the insulation housing is also provided with washinggrooves which extend fully across the width of the package and areparallel to the sides containing input and output terminals and arelocated between the terminals and the pad. These grooves increase thesurface tracking distance between the terminals and the pad and allowwashing out of solder flux during solder down.

In accordance with a further feature of the invention, short shallowshelves extend from the bottom ends of the grooves and across the widthof the housing bottom to improve the flux washing function.

As previously described, the various terminal pins are partly verticallysheared or offset to improve the plastic lock. In accordance with afurther feature of the invention, the partially rounded edge of theoffset region is provided with a small square notch or stepped corner togive a sharp edge to prevent the bleeding of plastic over the bottomsurfaces of the terminal during molding.

As a further feature of the invention, the terminals are formed withelongated crushable beads at their side surfaces adjacent the portionsof the terminals just outside the plastic housing. These beads arecrushed inwardly by the molding tool when it closes, to provide a sealwhich prevents the molding plastic from bleeding out and over the sidesof the terminals which extend beyond the housing and which couldinterfere with solder connection to the terminals.

As a still further feature of the invention, an integral lead frame barconnects the power input terminals at the two corners of the housing andinternally of the housing. Wire bonds from the die within the housingare made to this single bar which is contained within the housing. Thebar improves the wire bond connection and also acts as a plastic lock tothe housing.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawing a form which is presently preferred, it being understood,however, that the invention is not limited to the precise arrangementand instrumentality shown.

FIG. 1 is a top view of the package of the preferred embodiment of theinvention.

FIG. 2 is a bottom view of the package of the preferred embodiment ofthe invention.

FIG. 3 is a side view of the package of the preferred embodiment of theinvention.

FIG. 4 is an end view of the main power terminal end of the package ofthe preferred embodiment of the invention.

FIG. 5 is a cross-sectional view of an IMS support board to which thepackage of FIGS. 1 to 4 can be mounted.

FIG. 6 is a top view of the lead frame used in the package of FIGS. 1 to4.

FIG. 7 is a cross-sectional view of FIG. 6 taken across section line7--7 in FIG. 6.

FIG. 8 is a bottom view of the lead frame of FIG. 6.

FIG. 9 is an enlargement of circled area "A" in FIG. 7.

FIG. 10 is an enlarged detail of circled region "B" in FIG. 3 showingthe locking of a control terminal into the plastic housing and the novelwashing groove structure.

FIG. 11 shows a portion of the flat single gauge lead frame of thepreferred embodiment of the invention without terminal lead offset.

FIG. 12 shows the lead frame of FIG. 11 after offsetting of the terminalleads.

FIG. 13 shows the formation of a stepped corner at corner region "C" inFIG. 12 to prevent plastic bleeding over the bottom surface of theterminal during molding.

FIG. 14 is an enlargement of circled region "D" in FIG. 6 and shows aplastic lock groove extending from the end of the plastic lock slot inthe lead frame.

FIG. 15 is a cross-section of FIG. 14 taken across section line 15--15in FIG. 14.

FIG. 16 is a top view of a severed lead frame terminal and showssacrificial vertical crushable bumps or beads on the sides of theterminal which seal the mold tool to prevent bleeding of the plasticonto the exposed solderable surface of the terminal.

FIG. 17 is a side view of FIG. 16.

FIG. 18 shows the lead frame of FIG. 6 with semiconductor die soldereddown to the pad and bonding wire connecting the die to externalterminals, and shows the manner in which the lead frame is trimmed afterthe molded housing (not shown) is formed.

FIG. 19 shows a circuit diagram of FIG. 18.

FIG. 20 shows a prior art structure in which sacrificial beads are notprovided on the terminals which extend through the plastic housing.

FIG. 21 shows the terminal of FIG. 20 in perspective view withsacrificial protrusions.

FIG. 22 shows the sacrificial protrusions after molding.

FIG. 23 shows the mold and terminal before molding.

FIG. 24 shows the mold and terminal after molding.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring first to FIGS. 1 to 4, the exterior of the surface-mountpackage of the preferred embodiment of the invention is shown, andconsists of a molded insulation plastic housing 30 which is an elongatedrectangle and incorporates the upper surfaces and portions of the edgesof a flat single gauge lead frame which may be a conventional copperalloy about 1.27 mm thick. In a preferred embodiment, housing 30 isabout 29 mm long, 14.2 mm wide, and 4.27 mm high. The lead frame isdescribed in more detail below with reference to FIGS. 6, 7 and 8. Thelead frame elements shown in FIGS. 1 to 4 are lead frame heat sink pad31, power terminals 32 and 33 on the corners of one edge of therectangular housing 30 and control terminals or pins 34, 35 and 36 alongthe opposite side of housing. Terminals 32 to 36 extend about 1 mmbeyond the ends of the housing. Terminals 34 and 35 are preferablyclosely spaced, for example, about 2.5 mm center to center whileterminals 35 and 36 are preferably more widely spaced, for example,about 6.0 mm center to center.

As shown in FIG. 3, the bottom surfaces of pad 31 and terminals 32 to 36are co-planar and are capable of connection to the patterned surface ofa heat sink support such as an IMS board. FIG. 5 shows a cross-sectionof a typical IMS board which consists of a thick thermally conductive(copper or aluminum alloy) substrate 40 which is covered with a verythin insulation polymer 41. A patternable thin conductive solderablelayer 42 is disposed atop insulator 41. Any desired pattern can beformed in layer 42 but, in FIG. 5, the layer 42 is separated into aportion 42a and a plurality of segments aligned with terminals 32 to 36.Only segments 42b and 42c, which align, for example, with terminals 32and 34 respectively, are shown in FIG. 5. It then becomes convenient tosolder down the bottom of the package of FIGS. 1 to 4 to the IMS boardof FIG. 5, using standard soldering techniques.

To assist and improve the solder-down operation, flux washing grooves 50and 51 (FIGS. 2 and 3) are formed across the bottom of plastic housing30 between and parallel to the opposite edges of pad 31 and the lines ofterminals 32-33 and 34-36 respectively. Grooves 50 and 51 preferablyhave a curved cross-section and a radius of about 0.4 mm. These groovesare very useful to assist in washing away solder flux after the solderdown operation, and further increase the tracking distance over theplastic surface between pad 31 and terminals 32 to 36.

In order to improve the flux washing function, it was found useful toprovide shallow short shelves, shown as shelves 60 and 61 in FIG. 10.Shelves 60 and 61 have a depth of about 0.1 mm, and ensure that theirrespective washing grooves 50 and 51 are spaced and open above thesubstrate to which the package is soldered.

The lead frame itself, before receiving die or a housing, is shown inFIGS. 6, 7 and 8. The pad 31 and terminals 32 to 36 are integral partsof the lead frame and are joined by segments which are cut away afterover-molding to isolate contacts 34 to 36 from contacts 32, 33 and fromthe pad 31 and from one another. The lead frame also contains a heavycross bar 70 (FIGS. 6 to 8 and 18) which connects power terminals 32 and33 together, and further acts as a plastic lock to help anchor the leadframe into plastic housing 30. Bar 70 also serves as a bonding surfacefor wire bonds, as will be described in connection with FIGS. 18 and 19.

Pad region 31 has two parallel thin slots 71 and 72 (FIGS. 6 to 8, 14and 15) which fill with plastic during the molding operation, alsocreating a plastic lock, to help lock pad 31 to housing 30. Intentionalshort barbs 73 and 74 extend from the interior walls of slots 71 and 72,respectively, creating yet another plastic lock, to further anchor thelead frame pad 31 to the plastic housing.

In order to still further assist as a plastic lock, dovetail grooves 80to 83 (FIGS. 6, 14 and 15) extend from the ends of slots 71 and 72 onthe top surface of pad 31 toward the ends thereof. These fill withplastic during the molding operation to further lock pad 31 to thehousing.

It will be noted that the top central surface of pad 31 has a "waffled"surface 85. The top surface of pad 31 may be nickel plated and has apattern of shallow (preferably about 0.05 mm) spaced indentations,preferably dot-shaped indentations of about 0.25 mm diameter on about0.6 mm centers. It is known that this waffle pattern improves the solderdown of die to the waffled surface. In accordance with another aspect ofthe preferred embodiment of the invention, the opposite side of the pad31 is also provided with a waffle pattern 86 (FIG. 8). This surface isordinarily flat and smooth, but it has been found that if the surface isslightly concave, undesirable solder voids can be formed during thesolder-down process. In accordance with the invention, a waffle patternon the bottom of the concave lead frame surface improves its ability tobe soldered to a flat heat sink surface by increasing the wetting andflow of solder therebetween.

FIG. 11 shows a cross-section of a portion of the lead frame containingpad 31 and contact 36. This frame is originally a perfectly flat framewith flat planar top and bottom surfaces. It was found that, by slightlydisplacing the terminal sections of the lead frame by a partial stampingoperation as shown in FIG. 12, the terminals are better locked into theplastic housing 30. The actual displacement used is about 0.5 mm for a1.27 mm thick lead frame. It was found that during the molding operation(after this off-setting process), plastic tended to bleed past theslightly rounded edge at location "C" in FIG. 12, over the bottomsurface of terminal 36 and over the other off-set terminals 32 to 35. Itwas found that the stamping of a square notch 95 (FIG. 9 and 13) at thecorner "C" in each terminal prevented this undesired bleeding ofplastic. The notch 95 is preferably formed by shearing the material (asopposed to bending) because such a method requires no additional lateralspace.

Notch 95 is about 0.2 mm deep and 0.3 mm long. Notch 95 is shown inFIGS. 6 and 7 for each of the offset terminals 32 to 36.

As a still further feature of the novel package, each of the pins orterminals extending through the plastic housing 30 has one or more smallbeads extending from the thickness of the terminal. The thickness of thebeads 100, 101 should vary in accordance with the thickness of the leadframe. In general, the beads 100, 101 will be from 0.05 to 0.5 mm thick.Thus, as shown in FIGS. 16 and 17, two crushable beads 100 and 101,having radii, for example, of about 0.2 mm (for a lead frame of 1.2 mm),serve to be crushed or partly flattened by a molding tool to prevent thebleeding of plastic beyond the boundary defined by the beads 100 and101. The crushable beads are shown in FIGS. 6 and 7 at all necessarylocations on the lead frame.

FIGS. 16, 17 and 20 to 24 show added detail with respect to thecrushable beads 100 and 101 on illustrative terminal 36. Thus, referringto FIG. 20, it was found that, during molding of housing 30, there wasan uncontrolled flow of a thin sheet of plastic 120, 121 on the sides ofthe terminals, such as terminal 36 outside the mold housing 30. Thisplastic must be removed since it interferes with soldering to terminal36.

This problem was solved by the crushable beads 100 and 101, shown inFIGS. 16, 17 and 21, on the sides of terminal 36. Thus, during molding,beads 100 and 101 are crushed by the mold, as shown in FIG. 22 to act asa dam against the outward leakage or bleeding of plastic, as shown. Itwill be noted that trapezoidal flats are crushed into beads 100 and 101.Thus, as shown in FIGS. 23 and 24, the mold 140 has a tapered channelportion 141 which receives the terminal 36. As the mold closes from theposition of FIG. 23, terminal 36 is forced into the tapered channel 141which flattens the beads 100 and 101 with flats 150 and 151respectively. Thus, during molding, the crushed beads seal channel 141to prevent bleeding of plastic past the crushed regions of beads 100 and101. The divergent angle of channel 141 permits the easy release ofterminal 36 (and of all terminals 32 to 36) after molding is completedas in FIG. 24.

FIG. 18 shows the lead frame pad 31 after two semiconductor device die110 and 111 have been soldered down to pad 31, called a copack. Die 110and 111 of the copack may be of any type, but are shown in FIG. 18 and19 as a power IGBT and a fast recovery diode (FRED), respectively.

It is noted that in FIG. 19 the collector electrode of IGBT 110 isconnected to the cathode of FRED diode 111 because those electrodes aresoldered to and connected by conductive pad 31. Thus, the conductive pad31 provides a means for electrically interconnecting the copack to anexternal circuit. The top emitter electrode of IGBT 110 is wire bonded,as by wires 112 to the anode electrode of FRED diode 111. Wire bonds 113continue and are connected to cross bar 70 and terminals 32, 33.

Further, a wire bond 115 is made from the gate pad of IGBT 110 to gateterminal 35 and an emitter Kelvin connection 116 may also be provided atterminal 34 as shown in FIG. 18.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

What is claimed is:
 1. A surface mount semiconductor device package, comprising:a semiconductor device; a metal pad on which the semiconductor device is mounted; a housing formed of a flowable material which bonds to the metal pad and encapsulates the semiconductor device when it cures; at least one terminal being spaced away from and co-planar with the metal pad, the terminal having spaced apart side surfaces which define a thickness of the terminal; and at least one crushable bead disposed on each side surface of the terminal, each bead having a thickness corresponding with the thickness of the terminal and being sized and positioned such that a portion thereof is deformed by and conforms to a channel of a molding device when it receives the terminal to form the housing, the bead preventing the flowable material of the housing from bleeding into the channel of the molding tool.
 2. The surface mount semiconductor device package of claim 1, wherein the package includes a plurality of terminals, each terminal having a crushable bead on each side surface thereof.
 3. The surface mount semiconductor device package of claim 2, wherein at least one crushable bead on one of the terminals is axially aligned with at least one crushable bead on the other terminals which are located on one side of the package.
 4. The surface mount semiconductor device package of claim 3, wherein the crushable bead is from about 0.05 to 0.5 mm thick.
 5. The surface mount semiconductor device package of claim 1, wherein the crushable bead includes a rounded portion having a radius of about 0.2 mm.
 6. The surface mount semiconductor device package of claim 1, wherein the crushable bead includes a trapezoidal flat formed by the channel of the molding device when it deforms the bead.
 7. The surface mount semiconductor device package of claim 6, wherein the molding device has a tapered channel portion which receives the terminal and the bead and flattens the bead when the mold closes. 